SG-2000 MANUAL
42
The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
©1995, SGC, Inc.
TEL: (206) 746-6310 FAX: (206) 746-6384
There are also some other effects on antenna performance when it is raised up from
ground. At low levels, down close to the ground, an antenna will exhibit a certain
impedance (usually lower than at higher elevations). As a dipole antenna is moved up
from the ground, it will go to a higher impedance until it settles at between 50 and 72
ohms at one wavelength over the ground.
Radio signals are polarized. This means that a signal coming from a vertical antenna
will be best received by a vertical antenna, provided that the antennas are of equal gain.
This means that even though a quarter wavelength vertical will likely be heard at
distance better on a similar vertical than on a horizontal dipole antenna, a horizontal
antenna array with substantial gain, such as a log periodic beam, will receive the signal
better than either the vertical or horizontal dipole. Antenna gain is the best investment
you can make because it works for you both in the transmit mode and in the receive
mode.
A couple of other comments about polarization: most of the noises which you hear
from over the horizon are vertically polarized so a horizontally polarized antenna will
be quieter. And there are, in addition, circularly polarized antennas such as large delta
loop antennas on low frequencies and quad type loops on the higher HF frequencies.
11.2
Near Vertical Incidence Skywave
Not all HF SSB work is done at transcontinental or intercontinental range. There is
often a need to operate at close-in distances of less than 400 km. HF can provide a good
solution for this type of communications. The technique is called Near Vertical Incident
Skywave propagation (NVIS).
The NVIS antenna works on principles opposite those which lead to good long distance
communications. Instead of sending maximum signal toward the horizon to hit the
ionosphere at a shallow angle hundreds of miles away, the objective in NVIS antennas
is to bounce almost straight up and come back down. While this antenna works great in
the daytime and at short distances, it is very poor at night or when longer distances are
being attempted.
NVIS antennas are generally low frequency antennas which operate from 2 to 7 MHz.
The effect on performance of NVIS and groundwave radiation at these frequencies can
only be established over a period of several days and several nights of operating.
11.3
Base Station and Fixed Antennas
With the discussion of some basic antenna principles in mind, you must decide what
kind of antenna to install for a fixed station. If distances to be communicated are long,
say over 1,000 Km, then a low angle of radiation is desired. The antennas which work
best for these purposes are either vertical antennas, dipoles oriented so that their major
lobe is in the desired direction (dipole lobes are perpendicular to the wire axis), or beam
antennas.